1. Field of the Present Invention
The present invention relates generally to an inverter circuit for a discharge tube for use in an LCD unit, and, more specifically, to an inverter circuit for a discharge tube, which ensures high power efficiency.
2. Description of the Related Art
Some conventional inverter circuits for a discharge tube operate such that the primary side of a transformer is driven by a resonance frequency of a resonance circuit at the secondary side of the transformer, which comprises a leakage inductance and a parasitic capacitance of a discharge tube connected as a load. Such an inverter circuit is disclosed in U.S. Pat. No. 6,114,814.
This drive by the resonance frequency involves a phase difference between voltage and current at the primary side of the transformer, so that power efficiency of the transformer is not necessarily satisfactory.
There is another problem that high-order resonance frequencies existing at the secondary side of the transformer cause an accidental operation that influences undesirably its operation, which gives difficulty in designing the transformer.
The present invention has been made in view of the above problems. It is therefore an object of the present invention to provide an inverter circuit for a discharge tube that has an increased efficiency of a transformer and that is free from the influence of the high-order resonance frequencies.
In order to achieve the above object, it is noted that an excellent power efficiency can be obtained when the transformer is driven at a specific frequency range where the phase difference is small between voltage and current at the primary side of the transformer.
According to a first aspect of the present invention, an inverter circuit for a discharge tube comprises: a transformer which includes a resonance circuit composed of a parasitic capacitance of the discharge tube; and a H-bridge circuit which drives a primary side of the transformer at a frequency that is lower than a resonance frequency of the resonance circuit and that involves a phase difference between a voltage and a current at the primary side of the transformer, the phase difference falling within a predetermined range from its minimum point. Accordingly, the inverter circuit improves the power efficiency of the transformer, and suffers from little influence by the high-order frequencies, easing the transformer design.
According to a second aspect of the present invention, the predetermined range is preferably below the resonance frequency at the secondary side of the transformer and covers xe2x88x9230xc2x0 from the minimum point. Accordingly, the inverter circuit improves reliably the power efficiency of the transformer.
According to a third aspect of the present invention, the inverter circuit for a discharge tube may further comprise a burst circuit that outputs a predetermined burst signal, whereby the primary side of the transformer is driven intermittently. Accordingly, light is modulated easily over a wide range.
According to a fourth aspect of the present invention, the burst circuit outputs an inputted pulsed signal as a burst signal when a resistance that determines an oscillating frequency is set to be higher than a predetermined value, and outputs a burst signal obtained from a predetermined DC signal and an oscillated triangular wave when the resistance is set to be lower than a predetermined value. Accordingly, the inverter circuit outputs easily a plurality of burst signals.
According to a fifth aspect of the present invention, when the burst signal goes high, an inverting input terminal of an error amplifier which feedback-controls a current of the discharge tube is pulled up, whereby the primary side of the transformer is inactivated. Accordingly, light is modulated easily and reliably over a wide range.
According to a sixth aspect of the present invention, the H-bridge circuit is composed such that two series circuits each comprising a PMOS and an NMOS are connected to each other in parallel, and a delay circuit is connected to gate circuits of the PMOSs. Accordingly, the PMOSs and NMOSs in the series circuits are prevented from turning on simultaneously, thereby preventing malfunction and protecting circuits
According to a seventh aspect of the present invention, the H-bridge circuit is composed such that two series circuits each comprising a PMOS and an NMOS are connected to each other in parallel, and gates of two PMOSs are caused to rise at respective two points which correspond to the maximum peaks of a predetermined triangular wave output and which appear alternately with each other while gates of two NMOSs are caused to rise at respective two points which correspond to the minimum peaks of the triangular wave output and which appear alternately with each other. Accordingly, it is possible to generate an appropriate signal that is effective not to turn on PMOSs and NMOSs of the H-Bridge circuits simultaneously.
According to an eighth aspect of the present invention, the H-bridge circuit is composed such that two series circuits each comprising a PMOS and an NMOS are connected to each other in parallel, gates of two NMOSs are caused to fall at respective two points which correspond to crossings defined by ascending portions of a predetermined triangular wave output and a voltage output of the error amplifier and which appear alternately with each other, and gates of two PMOSs are caused to fall lagging behind falling of the gates of the two NMOSs. Accordingly it is possible to ensure that PMOSs and NMOSs are not caused to turn on simultaneously.
According to a ninth aspect of the present invention, a voltage feedback error amplifier is further provided for feedback-controlling an output voltage of the transformer. Accordingly it is possible to provide a constant open voltage of the transformer even in case of no or poor connection of a discharge tube to the output terminal of the transformer.
According to a tenth aspect of the present invention, a protection circuit is further provided for inactivating the H-bridge circuit when an output voltage of the error amplifier exceeds a predetermined value. According it is possible to prevent an overcurrent from flowing in the discharge tube or an overvoltage from being applied to the discharge tube.
According to an eleventh aspect of the present invention, a protection circuit is further provided for inactivating the H-bridge circuit when an output of the voltage feedback error amplifier exceeds a predetermined value. Accordingly, it is possible to ensure that any damages to the transformer or any circuits are prevented.
According to a twelfth aspect of the present invention, a protection circuit is further provided for inactivating the H-bridge circuit when an output of the transformer exceeds a predetermined value.
According to a thirteenth aspect of the present invention, the predetermined value defined in the eighth aspect of the present invention is a reference voltage of a comparator of the protection circuit.